Instabilities in tidal turbine wakes

We report on the observation of vortex instabilities in the wake of a tidal turbine undergoing axially oscillating inflow, using unsteady RANS simulations. Unsteady inflow causes the turbine blades to shed vorticity of time-varying strength into the wake, which in turn leads to a spatial variation in wake vortex strength, such that the vorticity of adjacent returning wake segments can differ. This spatial variation triggers the instability of tip vortices, the characteristics of which are shown to be governed by the’frequencyratio’ω/ΩNB (where ω is harmonic inflow frequency, Ω is turbine rotational frequency and NB is the number of blades). Ifω/ΩNB=m, where m is an integer, the wake is stable. Ifω/ΩNB takes the form of 1/mor (m−1)/m, however, m adjacent tip vortices start leapflogging and merging into a larger vortex, creating a new vortex street with a lower spatial frequency. At other frequency ratios, the tip vortices exhibit more irregular deformation, suggesting a possible early breakdown into turbulence. This has implications for both wind and tidal farm design, where the interaction of downstream turbines with the wakes of upstream turbines is an important consideration.